# Crank-Nicolson Method for Solving 1D Schrödinger Equation

To solve a one-dimensional time-dependent Schrödinger equation numerically, consider a difference method.

The equation to be solved is

$\mathrm{i}\hbar\frac{\partial \psi(x, t)}{\partial t}=-\frac{\hbar^2}{2m}\frac{\partial^{2} \psi(x, t)}{\partial x^{2}}+V(x, t) \psi(x, t).$

Discretise the $x$, and denote $x_j=j\Delta x,\ j=1,\cdots,L$ and $\psi(x_j)=\psi_j$ To perform the difference, define the discretised Hamiltonian $H_D$ as

$H_D\psi_j=-\frac{\hbar^2}{2m}\frac{1}{\Delta x^2}\left[\psi_{j-1}+\psi_{j+1}-2\psi_j\right]+V_j\psi_j,$

which can be viewed as a matrix performed on a vector $(\psi_1,\cdots,\psi_L)$. Discretise the time and denote $\psi(t_n)=\psi(n\Delta t)=\psi^n$. A trivial differential method is

$\mathrm{i}\hbar\frac{1}{\Delta t}(\psi^n_{j+1}-\psi^n_j)=H_D\psi^n_j,$

i.e.,

$\psi^{n+1}_j=\left(1-\mathrm{i}\Delta t H_D/\hbar\right)\psi^n_j.$

To check the stability of such a method, consider the analytical solution to the discretised equation for $V_j$ constant

$\psi^n_j=\xi^n\exp(\mathrm{i}kj\Delta x),$

which yields

$\xi=1-\frac{\mathrm{i} \Delta t}{\hbar}\left[\frac{\hbar^2}{2m}\frac{4}{\Delta^{2} x} \sin ^{2}(k \Delta x / 2)+V_{j}\right].$

For any $\Delta x$ and $\Delta t$, $|\xi|>1$, indicating that this method is always unstable.

A modification of this method is to replace the discretisation form by

$\psi^{n+1}_j=\psi^n_j-\mathrm{i}\Delta t H_D/\hbar\psi^{n+1}_j,$

i.e.,

$\psi_{j}^{n+1}=\sum_{j'}\left(1+\mathrm{i} \Delta t H_D/\hbar\right)_{jj'}^{-1} \psi_{j'}^{n}$

One can check that in this method

$\xi=\frac{1}{1+\frac{\mathrm{i} \Delta t}{\hbar}\left[\frac{\hbar^2}{2m}\frac{4}{\Delta^{2} x} \sin ^{2}(k \Delta x / 2)+V_{j}\right]},$

which indicates that this method is always stable. However, the time evolution $\left(1+\mathrm{i} \Delta t H_D/\hbar\right)^{-1}$ is still not unitary, so that it does not preserve the norm of the wave function.

A further modification is to set the time evolution as

$\frac{1-\mathrm{i}\Delta tH_D/2\hbar}{1+\mathrm{i}\Delta t H_D/2\hbar}.$

To evaluate the stability,

$\xi=\frac{1-\frac{\mathrm{i} \Delta t}{\hbar}\left[\frac{\hbar^2}{2m}\frac{4}{\Delta^{2} x} \sin ^{2}(k \Delta x / 2)+V_{j}\right]}{1+\frac{\mathrm{i} \Delta t}{\hbar}\left[\frac{\hbar^2}{2m}\frac{4}{\Delta^{2} x} \sin ^{2}(k \Delta x / 2)+V_{j}\right]},$

which ensure the stability for any choice of $\Delta x$ and $\Delta t$.

The third method is called Crank-Nicolson method. It is an implicit form and requires an inverse of a tridiagonal matrix, whose time cost is $\mathcal{O}(L)$. To calculate the time evolution, one need to perform matrix products, whose time cost is $\mathcal{O}(L^2)$.

The code for this method with a periodic boundary condition can be found at Florestan-Eusebius/Crank-Nicolson-Solver: The Crank Nicolson Solver to solve 1D time dependent Schrödinger equation. (github.com), where the harmonic oscillator is tested as an example. We see that result calculated by this method agrees with the theoretical result perfectly.